There is a greater variety in types of organic compounds compared to inorganic compounds, and materials that have a variety of different functions can be designed and synthesized. Because of such advantages, attention has been focused on electronics devices in which organic compounds are used in recent years. For example, solar cells, light-emitting elements, transistors, and the like in which organic compounds are used as functional materials are some typical examples of these electronics devices.
These electronics devices are devices that use the electrical properties and optical properties of organic compounds; of these devices, in particular, research and development of light-emitting elements in which organic compounds are used as light-emitting substances has been showing an impressive amount of progress.
The structure of these light-emitting elements is a simple structure in which only a light-emitting layer that contains an organic compound, which is a light-emitting substance, is provided between electrodes, and these light-emitting elements have been attracting attention as elements of next-generation flat panel display panels because of their characteristics such as a thin shape, lightweight, high response speed, and low voltage driving. Furthermore, displays in which these light-emitting elements are used also have the characteristics such as superior contrast and image quality and a wide viewing angle.
The light-emitting mechanism of the light-emitting elements in which organic compounds are used as light-emitting substances is carrier injection. In other words, by application of a voltage to a light-emitting layer interposed between electrodes, holes and electrons injected from the electrodes recombine to place the light-emitting substance into an excited state, and the light-emitting substance emits light when the light-emitting substance returns to the ground state from the excited state. Further, as the types of excited states, there can be a singlet excited state (S*) and a triplet excited state (T*). Furthermore, it is thought that the ratio of S* to T* in a light-emitting element is statistically 1:3.
At room temperature, a compound that converts a singlet excited state into luminescence (hereinafter referred to as a fluorescent compound) exhibits only luminescence from a singlet excited state (fluorescence), not luminescence from a triplet excited state (phosphorescence). Therefore, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is thought to have a theoretical limit of 25% on the basis that S*:T*=1:3.
On the other hand, by using a compound that converts a triplet excited state into luminescence (hereinafter referred to as a phosphorescent compound), internal quantum efficiency can be improved from 75% to 100% theoretically. That is, emission efficiency can be three to four times as high as that of a fluorescent compound. From such a reason, in order to achieve a light-emitting element with high efficiency, a light-emitting element using a phosphorescent compound has been actively developed recently.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound as described above, in order to suppress concentration quenching of the phosphorescent compound or quenching due to triplet-triplet annihilation, the light-emitting layer is often formed so that the phosphorescent compound is dispersed in a matrix formed of another substance. In that case, a substance serving as a matrix is referred to as a host material, a substance that is dispersed in a matrix, such as a phosphorescent compound, is referred to as a guest material.
When a phosphorescent compound is used as a guest material, a host material is needed to have triplet excitation energy (an energy difference between a ground state and a triplet excited state) higher than that of the phosphorescent compound. In Patent Document 1 (Japanese Published Patent Application No. 2002-352957), TAZ is used as a host material of a phosphorescent compound which emits green light.
However, there has been a problem in that TAZ has difficulty in accepting holes in exchange for having high triplet excitation energy and accordingly driving voltage increases. Therefore, a substance that has high triplet excitation energy and also can easily accept or transport both holes and electrons (i.e. a bipolar substance) is required as a host material for a phosphorescent compound.
Furthermore, because singlet excitation energy (the difference in energy between a ground state and a singlet excited state) is greater than triplet excitation energy, a material that has high triplet excitation energy will also have high singlet excitation energy. Consequently, a substance that has high triplet excitation energy is also useful in a light-emitting element formed using a fluorescent compound as a light-emitting substance.